A Cu—Zn based alloy has poor spring properties as compared with phosphor bronze, beryllium copper, Corson alloy or the like, but the alloy is inexpensive, and hence it is broadly used as an electric contact material for a connector, a terminal, a relay, a switch and the like. A typical example of the Cu—Zn based alloy is brass, and an alloy such as C2600 or C2680 is defined in JIS H3100. When the Cu—Zn based alloy is used as an electric contact material, the alloy is often plated with Sn so as to stably obtain low contact resistance. A large amount of Sn-plated Cu—Zn based alloy strips is used in electric and electronic components such as terminals of wire harness for car electric equipment, terminals of printed circuit boards (PCB) and connector contacts for households, because Sn is excellent in solderability, corrosion resistance and electric connection properties.
The Sn-plated Cu—Zn based alloy strip is manufactured by steps of forming a base plating layer by an electric plating process after degreasing and pickling, then forming an Sn plating layer by the electric plating process, and finally performing a reflow treatment to melt the Sn plating layer.
To plate the Cu—Zn based alloy with Sn, the alloy is usually subjected to base plating prior to the Sn plating. This is because if the alloy is not subjected to the base plating, Zn in an Cu—Zn alloy strip forms a Zn concentrated layer on the Sn-plated surface during the reflow treatment, and the solderability deteriorates. That is, the base plating is performed in order to obtain an underlayer in which the diffusion of Zn of the alloy strip in the Sn-plated surface is suppressed.
In order to improve the heat resistance of the Sn plating, the Cu—Zn based alloy is subjected to Cu/Ni double layer base plating as the base plating. In the above Cu/Ni double layer base plating, electric plating including Ni base plating, Cu base plating and Sn plating are performed in this order, followed by the reflow treatment. A plating film layer after the reflow has a constitution in which an Sn phase, a Cu—Sn phase and an Ni phase are deposited on the alloy strip in this order from the surface. Details of this technique are disclosed in Patent Documents 1 to 3 (JP6-196349A, JP2003-293187A, JP2004-68026A) and the like.
It is known that when an Sn-plated strip is left to stand at room temperature, single crystals of Sn grow from the Sn-plated surface. The single crystals of Sn are referred to as whiskers, which sometimes cause the short-circuit of the electronic components. The whiskers are generated owing to the internal stress of the Sn-plated film formed by electrodeposition. Therefore, the reflow treatment for melting Sn to remove the internal stress of the film is effective as means for suppressing generation of whiskers. In the manufacturing process of a Cu/Ni double layer base heat-resistant Sn-plated Cu—Zn alloy strip, the reflow is performed, so that a satisfactory resistance to whiskers is obtained.
[Patent Document 1] Japanese Patent Application Laid-Open No. 6-196349
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-293187
[Patent Document 3] Japanese Patent Application Laid-Open No. 2004-68026
However, a very large internal stress is locally applied to an electric contact portion such as the terminal. Therefore, micro whiskers are sometimes generated even in the reflowed Sn-plated strip which has been considered as having the satisfactory resistance to the whiskers. In recent years, owing to the multipolarization of connector or the like, a space between terminals becomes small, and hence even such micro whiskers as not to be heretofore any problem might cause a short-circuit of a circuit. As a result, even for the Cu/Ni double layer base heat-resistant Sn-plating of the Cu—Zn alloy which has been considered to have the satisfactory resistance to whiskers, further improvement of whisker resistance is required.